The invention relates to low-viscosity dental materials containing a non-settling nanoscale filler. In particular, it relates to low-viscosity dental varnishes, dental sealants, and dental bonding agents containing a nanoscale filler and forming a stable sol with said filler. The filler improves the mechanical properties of the dental materials, e.g. the abrasion resistance and the compressive strength, and also improves their performance, e.g. it reduces microleakage and increases bond strengths.
A filler that forms stable sols with low-viscosity dental materials is prepared by surface treatment of very fine materials with suitable agents. Complete incorporation of the filler into the low-viscosity dental materials is achieved by employing high shear strengths such as with sonication.
Fillers of various sizes and types of materials are widely used in dental materials such as dental composites, compomers and cements. In these materials, fillers are employed to improve mechanical properties such as compressive strength, abrasion resistance, surface hardness and the like. Sometimes combinations of different particles sizes of fillers are used [e.g. U.S. Pat. No. 5,356,951]. Often the surface of the fillers has been chemically modified to become more compatible with the matrix [B. Arkles, Chemtech 7, 766 (1977)].
These materials typically have a high viscosity and a high filler content. Therefore settling of the filler in the uncured material is only a minor problem.
Other applications in dentistry demand dental materials that, as their characteristic property, display a low viscosity. Typical examples for materials of this type are dental bonding agents [R. G. Craig, W. J. O""Brien, J. M. Powers, xe2x80x9cDental Materials, Properties and Manipulationxe2x80x9d, p. 77-78, Mosby-Year Book, St. Louis 1992] and dental varnishes. For optimum performance, these materials have to deeply penetrate the dentin. This is something that can only usually be achieved by materials with a low viscosity and good wetting properties. However, even with these materials, the clinical performance can be improved by increasing the hardness and mechanical strength of the cured material. Potentially, incorporation of filler into these low-viscosity dental bonding agents, dental varnishes and other dental materials can increase their mechanical strength. Nevertheless, these low-viscosity dental materials rarely contain filler.
The density of filler and the matrix material differs considerably. While most known fillers have a density of  greater than 2 g/ml (gram/milliliter), most matrix materials, e.g. solvents or resins, have densities of about 1 g/ml or below. Therefore, even if the polarity of the filler surface and of the matrix are compatible, some settling of filler occurs due to the difference in densities.
Raising the filler content up to a level where settling is impossible also leads to a drastic increase in viscosity which for the type of materials that have to penetrate the dentin to work properly, is not acceptable.
Therefore, there is a need for a filler that can be evenly distributed in a low-viscosity dental material to form a stable sol without drastically increasing the viscosity of the dental material.
This filler, if properly selected, will improve the physical properties of the low-viscosity dental materials with which it is employed.
It is an object of the invention to provide low-viscosity dental materials comprising a nanoscale filler. The nanofiller content in the low-viscosity dental materials improves properties that are clinically relevant for said materials. For example, for a protective dental varnish, the nanofiller content increases abrasion resistance and surface hardness. For a dental bonding agent, the nanofiller content increases adhesion to both enamel and dentin and improves marginal integrity.
The nanofiller provided by this invention has a mean primary particle size of about 1 nm to about 100 nanometers (nm). It is prepared from fine fillers such as glass, alumina, silica and the like by chemically modifying the surface in a non-aqueous solvent followed by drying. The filler is then incorporated into the low-viscosity dental material by applying high shear forces, e.g. by sonication. This incorporation leads to the nanofiller forming a stable, non-settling sol with the low-viscosity dental material.
These and other objects of the invention which shall be apparent from the specification to follow, are accomplished by the invention as hereinafter described and claimed.
In general, a dental material comprises a nanoscale filler having a primary particle size of from about 1 nm to about 100 nm. The filler may be selected from the group selected of ground glass, ground quartz, highly dispersed silica, zeolite, laponite, kaolinite, vermiculite, mica, ceramic metal oxides, alumina, pyrogenic silica, sparingly volatile oxides of titanium, zirconium, germanium, tin, zinc, iron, chromium, vanadium, tantalum, niobium, and mixtures thereof.
The present invention provides low-viscosity dental materials comprising a nanoscale filler. The nanofiller content in the low-viscosity dental materials improves properties that are clincially relevant for said materials.
A preferred range of nanofiller is from about 0.01 to about 20 percent by weight based upon 100 percent by weight of the dental material.
From about 10 to about 90 percent by weight of polymerizable materials are provided to form the polymeric network. Useful polymerizable materials include methacrylate and acrylate monomers having at least one saturated double bond, and mixtures thereof. Preferred polymerizable monomers are those that are curable, more preferably, light-curable.
The dental materials described in this invention may comprise solvents. Useful solvents include water, acetone, ethanol, ethyl acetate and other organic solvents with boiling points below that of water. A useful amount of solvent would be from about 10 to about 90 percent of weight of the dental material.
The dental materials may include resins, fillers other than nanoscale fillers, stabilizers, initiators, fluorides, solvents and other substances commonly used in dental materials.
The dental materials described in the present invention comprise polymerizable monomers and nanoscale fillers in a stable sol of low viscosity. The low viscosity allows deep penetration of the dentin, resulting in good adhesion to the dentin and mechanical strengthening of the dentin. The nanofiller particles incorporated into the dental materials enforces these properties. By xe2x80x9clow-viscosityxe2x80x9d as used herein, it is meant from about 0.0001 to about 1 Pas.
By xe2x80x9cnanoscale fillerxe2x80x9d or xe2x80x9cnanofillerxe2x80x9d it is meant materials having a primary particle size of from about 1 nm to about 100 nm. By xe2x80x9cprimary particle sizexe2x80x9d is meant that with powders, the primary particles are the smallest homogeneous particles. The term is used to determine primary from secondary particles that may form by agglomeration or aggregation of primary particles and are therefore necessarily larger than the primary particles. For example, with Aerosil 380 as discussed below, the primary particle size is approximately 7 nm but there may be agglomerates or aggregates of these primary particles having a larger size. Of course, these larger secondary particles are still within the scope of the invention. However, by xe2x80x9cprimary particlesxe2x80x9d is meant those that would remain after destruction of agglomerates and aggregates.
Examples of useful starting materials for the nanofillers described in the present invention include ground glass or quartz, highly dispersed silica, zeolite, laponite, kaolinite, vermiculite, mica, ceramic metal oxides, alumina, pyrogenic silica, sparingly volatile oxides of titanium, zirconium, germanium, tin, zinc, iron, chromium, vanadium, tantalum, niobium, and mixtures thereof. Preferred useful starting materials have to have a primary particle size of about 1 nm to about 100 nm.
For synthesis of the nanofiller described in the present invention, these materials are treated with an agent enabling the filler to form a stable sol in an organic solution with a viscosity of about 1 Pas. It is preferred to carry out this treatment in a non-aqueous solvent to prevent agglomeration of the filler particles.
Silanating agents are preferred, and it is further preferred to treat the filler before formation of the sol. Sol formation will be more fully described below.
Preferred silanating agents include those having at least one polymerizable double bond and at least one group that easily hydrolyses with water. Examples of such agents include 3-methacryloxypropyltrimethoxysilane, 3-methacryloxypropyldimethoxy-monochlorosilane, 3-methacryloxypropyldichloromonomethoxysilane, methacryloxypropyltri-chlorosilane, 3-methacryloxypropyldichloromonomethyl-silane, 3-methacryloxypropylmonochlorodimethylsilane, and mixtures thereof. These agents are preferably employed in a non-aqueous solution.
After drying of the filler materials treated with these agents, incorporation of the nanofiller is preferably done by mixing the nanofiller with the low-viscosity dental material and employing high shear strengths, e.g. with an Ultraturrax mixer or by sonication. As will be shown in the following examples, the nanofiller forms stable sols of low viscosity and improves relevant mechanical properties of dental materials. If used in a dental varnish, the nanofiller increases the abrasion resistance and the surface hardness. As a component of a dental bonding agent, the nanofiller improves bond strengths and marginal integrity of the bonding agent.
For some applications, thin films are necessary. For example, a cervical dental varnish should not be visible and therefore has to be thin. A colourless adhesive agent used to fix an inlay to tooth structure should be thin as no gap between the inlay and the tooth should be seen.
The lowest film thickness achievable with a given material depends on the viscosity of the material. Therefore low-viscosity film formers are preferable to achieve thin films. To get thin, hard films therefore fillers can only be employed if they do not significantly increase the viscosity of the film former and if their particle size is significantly lower than that of the film to be obtained. Nanofillers as described in Example 1 below meet these requirements and may therefore be employed to obtain thin, hard films. Thin films according to the invention have a film thickness of from about 1 to about 50 nm.